Methods for manufacturing unleaky closed containers and leak testing apparatus

ABSTRACT

In a leak testing apparatus and a method for manufacturing unleaky closed containers having first and second flexible wall areas with different flexibility characteristics, a biasing member is moved toward and onto the first flexible area and a force detector monitors a biasing force at the second flexible area. A container is considered unleaky if a difference signal from force measuring signals at first and second points in time fulfills a test criteria. Sampling the biasing force monitored which results in the first force measuring signal includes determining maximum biasing force signal value which occurs during a time span up to and including the first point in time.

TECHNICAL FIELD

The present invention departs from a technique of manufacturing unleakycontainers as disclosed in the WO 00/073760 or U.S. Pat. Nos. 6,557,395,6,439,032 or 6,840,087 all of the same applicant as the presentinvention.

SUMMARY

Thereby for manufacturing unleaky containers which have a first flexiblewall area and a second flexible wall area of different flexibilitycharacteristics the present invention is a manufacturing method whereinafter having provided a closed container, at least one biasing member ismoved towards and onto one of the first and second flexible wall areasof the container. Such biasing moving is stopped. A biasing force on thecontainer is monitored and the biasing force as monitored is sampledresulting in a first force measuring signal at a first point in time.The addressed biasing force as monitored is further sampled at least atone second subsequent point in time which results in a second forcemeasuring signal. A difference signal is generated in dependency of thefirst and second force measuring signals. The addressed container isconsidered unleaky if the difference signal fulfils a test criterion.Thereby the biasing member is moved towards and onto the first flexiblewall area of the container and monitoring the biasing force is performedat the second flexible area. Sampling of the biasing force monitoredwhich results in the addressed first force measuring signal comprisesdetermining a maximum force signal value having occurred during a timespan up to and including the first point in time.

In one embodiment of the addressed method the biasing member is moved upto a predetermined position with respect to the container which is inone embodiment defined by a mechanical stop. Further in one embodimentstopping of the biasing member is performed at least substantially atthe first point in time thus at least substantially at that moment atwhich sampling of the biasing force monitored results in the first forcemeasuring signal.

In one embodiment a predetermined time span is selected and the maximumforce value having occurred during this predetermined time span up toand including the first point in time is determined.

In one embodiment biasing comprises moving at least two biasing memberstowards and onto the first flexible wall area from opposite sides of thecontainer.

In one embodiment the first flexible wall area of the container is awall area of a body of the container and the second flexible wall areais a sealing cover of an opening of the container body.

Thereby in one embodiment the second flexible area is a foil-likesealing cover of the addressed opening.

In a further embodiment monitoring the biasing force at the secondflexible wall area is performed along a force sensing surface which isspaced from the second flexible area by a predetermined amount, saidsecond flexible wall area being thereby considered at unbiased conditionof the container. This predetermined amount is substantially smallerthan a maximum distance which the second flexible wall area may at allbow outwards due to an increased pressure in the closed container.

In one embodiment monitoring the biasing the force comprises monitoringby means of a resistance gauge.

In a further embodiment of the addressed method the biasing force asmonitored is compared at a third point in time previous to said firstpoint in time with a threshold value and a container is established ashaving a large leak if the force monitored does not at least reach thethreshold value.

In a further embodiment of the manufacturing method a multitude ofcontainers is provided, moving on a conveyor and moving the biasingmembers, stopping same, monitoring the biasing force, performing theaddressed samplings, generating the difference signal and furtherperforming the addressed leaky/unleaky consideration is performed onmore than one of the moved containers on the conveyor at leastsubstantially simultaniously.

In a further embodiment the force monitored is compared at a third pointin time previous to the first point in time with a threshold value andthere is established a container as having no large leak if the force asmonitored at the third point in time at least reaches the thresholdvalue. The force value monitored at the addressed third point in time ifthe threshold value is at least reached is averaged with such forcevalues generated at previously tested containers which have beenconsidered as having no large leak and the threshold value is applied independency of a result of such averaging.

In a further embodiment the difference signal is compared with asmall-leakage-indicative threshold value.

Still in a further embodiment the difference signal is averaged withsuch difference signals which have been generated during previoustesting of containers having been considered as unleaky, whereby thesmall-leakage-indicative threshold value is controlled in dependency ofthe result of such averaging.

Still in a further embodiment there is provided at least one forcethreshold value and the force monitored is compared with such thresholdvalue whereby the addressed threshold value is updated as a function ofcomparing result.

The various embodiments as defined above with their specific featuresmay be combined thereby defining for still further embodiments of themethod for manufacturing closed unleaky containers with accordinglycombined features.

The leak testing apparatus according to the present invention for leaktesting a closed container with at least a first and a second flexiblewall area of different flexibility characteristics comprises a biasingarrangement for compressing a container under test. It further comprisesa force detector which is applicable to the wall of the container undertest and which generates an electric output signal. The output of theaddressed force detector is operationally connected to a storing unit,the output of which being operationally connected to one input of acomparator unit, the second input thereof being operationally connectedto the output of the force detector. The biasing arrangement ispositioned so as to bias the first flexible area of the container andthe force detector is positioned to cooperate with the second flexiblearea of the container.

In one embodiment of the apparatus the biasing arrangement comprises atleast two relatively movable biasing members relatively movable in aplane. Thereby the force detector has a force sensing surface whichdetects forces substantially perpendicular to the addressed plane.

In a further embodiment of the apparatus the force detector comprises aresistance gauge.

Still in a further embodiment the biasing arrangement cooperates with amechanical stop limiting its biasing action upon the container.

Still in a further embodiment of the apparatus the output of the forcedetector is operationally connected to an input of a maximum valuedetecting unit.

Still in a further embodiment of the apparatus according to the presentinvention it comprises a conveyor arrangement for a multitude of theaddressed containers. At least two of the addressed biasing arrangementand force detector are provided moving with the conveyor.

The various embodiments of the apparatus may thereby be combinedresulting in further embodiments of such apparatus with combinedfeatures.

There is further provided a method for manufacturing unleaky closedcontainers with a first and a second flexible wall area of differentflexibility characteristics wherein a closed container is provided andat least one biasing member is moved relatively towards and onto one ofthe flexible areas of the container. The addressed moving is stopped.The biasing force on said container is monitored. The biasing force asmonitored is sampled which results in a first force measuring signal ata first point in time. The biasing force monitored is sampled at leastat one second subsequent point in time which results in a second forcemeasuring signal. A difference signal is generated in dependency of thefirst and the second force measuring signals as a leak indicativesignal. An average signal of difference signal as generated duringprevious testing of containers is updated with the actual differencesignal, if the container actually under test is unleaky. The differencesignal is thereby compared with at least one threshold signal whichthreshold signal is controlled in dependency of the addressed averagesignal. Thereby moving the biasing member is performed relativelytowards and onto the first flexible wall area and monitoring the biasingforce is performed on the second flexible wall area. Sampling thebiasing force monitored which results in the first force measuringsignal comprises determining a maximum force signal value which hasoccurred during a time span up to and including the first point in time.

Still in a further embodiment there is provided a method formanufacturing unleaky closed containers with a first and with a secondflexible wall area of different flexibility characteristics. Thereby aclosed container is provided and at least one biasing member is movedrelatively towards and onto one of said flexible wall areas of thecontainer. The moving is stopped. A biasing force on the container ismonitored. The biasing force as monitored is sampled which results in afirst force measuring signal at a first point in time. The biasing forcemonitored is further sampled at least at one second subsequent point intime which results in a second force measuring signal. A differencesignal is generated in dependency of the first and second forcemeasuring signals as one leak indicative signal. The biasing force asmonitored is further sampled at a further point in time resulting in anactual further force measuring signal which is leak indicative. Anaverage signal of further force measuring signals is generated duringpreceeding testing of unleaky containers and such averaged signal isupdated with the actual further force measuring signal if the actualfurther force measuring signal indicates a unleaky container. Thedifference signal is thereby compared with a thresehold value whichdepends from the addressed average signal. A container which isindicated as leaky is rejected.

Movement of the biasing member is performed relatively towards and ontothe first flexible wall area and monitoring the biasing force isperformed on the second flexible area. Sampling of the biasing force asmonitored which results in the first force measuring signal comprisesdetermining a maximum force signal value having occurred during a timespan up to and including the first point in time.

Still in a further embodiment there is provided a method formanufacturing closed containers with a flexible wall portion wherein aclosed container is provided and is biased. A biasing force on thecontainer is monitored and from such force as monitored a maximum forcevalue as occurring during a time span is detected. A signal whichdepends on the addressed maximum force value as detected is stored andcompared with a signal dependent on the biasing force as monitored. Thecontainer is rejected as leaky in dependency of a result of theaddressed comparing.

BRIEF DESCRIPTION OF DRAWINGS

The invention shall now be further exemplified with the help of figures.These figures show:

FIG. 1 schematically and simplified a closed container being leak testedin the frame of manufacturing such containers being unleaky andaccording to the present invention;

FIG. 2 an enlarged area of the representation according to FIG. 1showing biasing force monitoring at one of the flexible areas of thecontainer's wall according to FIG. 1;

FIG. 3 qualitatively different force versus time characteristics atcontainers tested with an apparatus according to the present inventionand by a testing procedure in the frame of the manufacturing methodaccording to the present invention;

FIG. 4 a simplified signal flow/functional block diagram of an apparatusaccording to the present invention operating according to the leaktesting procedure within the frame of the manufacturing method accordingto the present invention;

FIG. 5 an embodiment for accurately performing digital signal comparisonas applicable at the apparatus according to FIG. 4;

FIG. 6 different courses of force dependent signals over time asencountered at equally unleaky equal containers and as caused e.g. bymanufacturing tolerances or varying environmental parameters;

FIG. 7 an embodiment for generating a time varying threshold value forlarge leak detection;

FIG. 8 qualitatively courses of time varying reference and thresholdvalue signals as exploited in some embodiments of the present invention;

FIG. 9 by means of a simplified functional block diagram evaluating froma leak indicative signal whether a container under test has a small leakor not;

FIG. 10 for an embodiment of the present invention generating a timevarying threshold value for a small leak indication;

FIG. 11 in a simplified and schematic representation, an inline leaktesting apparatus according to the present invention for high-ratecontainer testing as applied in the frame of the manufacturing methodaccording to the present invention, finally selecting only unleakycontainers out of a stream of closed containers;

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1 shows schematically the principal according to the presentinvention. A closed container 1 to be leak tested e.g. within aproduction line for finally manufacturing exclusively unleakycontainers, has a first area 3 a of its overall wall 3 which isflexible. The opening 4 of container 1 is sealingly closed by a sealingfoil-like member, which is a second flexible area 3 b of the container'swall 3. The areas 3 a and 3 b are of different flexible characteristic.As an example and most common the container 1 is a bottle the bottlewall 3 c of which being of a plastic material the opening of which beingsealed with a foil-like cover 4 which is sealed to the border of theopening 4 of bottle wall 3 c e.g. by welding. The foil-like cover is onone hand and as was addressed flexible but substantially non-elastic asmade of a metalized plastic foil or a plasticized metal foil as ofaluminum. In opposition to this second flexible area 3 b formed by theaddressed sealing foil-like cover the first flexible area 3 a ofbottle-wall 3 c is of thicker plastic material and is more elastic. Thusthe addressed first and second areas 3 a, 3 b of the overall container'swall 3 are of different flexibility characteristics.

For leak testing the container 1 is positioned between two biasingmembers 5 a and 5 b of a biasing arrangement 5. The biasing members 5 aand 5 b are relatively moved towards and from each other to provide abiasing load B on the first flexible area 3 a. Thereby in the embodimentas shown in FIG. 1 both members 5 a and 5 b are equally moved towardsand from each other and with respect to a mechanic machine reference 6e.g. a conveyor for the container 1. By moving the members 5 a, 5 btowards each other the container 1 is squeezed at the area 3 a to bowinwardly as shown in dashed line. Due to the increased pressure withincontainer 1 caused by the addressed squeezing biasing by biasingarrangement 5, the second flexible area 3 b formed by the sealingfoil-like member is bowing outwards as also shown in FIG. 1 and, in anenlarged representation, in FIG. 2. The outwards bowing second flexiblearea 3 b is thereby pressed against the sensing surface 9 a of a forcedetector 9 which is stationary with respect to the mechanical reference6 of the testing machine. The distance d between the second flexiblewall portion 3 b formed by the sealing foil-like member and the sensingsurface 9 a of the force detector 9 is selected much smaller than themaximum distance D the foil-like member may bow outwards due to anincreased pressure inside the container 1; in this respect FIG. 2 doesnot show the correct relation of d and D. For a circular area 3 b with aradius in the range of 1 or 2 cm the spacing d is selected e.g. to be0.5 mm. The effect of selecting the spacing d small is that bowingoutwards of the second flexible area 3 b is limited to such an extentthat the sealing link or weld 5 is substantially not mechanically loadedby tensile strength by the outwards bowing.

As further shown in FIG. 1 schematically, the relative movement of thebiasing members 5 a and 5 b to squeeze first flexible area 3 a isgenerated by respective drives 7 a and 7 b and is limited by respectivestops 8 a and 8 b.

In FIG. 3 a qualitative force versus time diagram explaining theinventive method as performed by the inventive apparatus, is shown. Attimes 0 according to FIG. 3 the biasing movement of the two biasingmembers 5 a and 5 b is initiated. Because the characteristic of movementi.e. acceleration and thus speed as generated by the drives 7 a and 7 bupon the biasing members 5 a and 5 b is known the time span for movingthe biasing members 5 a and 5 b up to the stops 5 a and 5 b of FIG. 1 isknown and shown in FIG. 3 by the time span up to t₁. When the biasingmembers 5 a and 5 b have contacted the first flexible area 3 a then,squezzingly bias that area 3 a the pressure within container 1,irrespective whether filled with a product or not, rises which leads tothe second flexible area 3 b formed by the sealing foil-like memberbeing bowed outwards. As soon as the gap with the spacing d is bridgedby the outbowing second flexible area 3 b and due to still increasingbiasing movement of the members 5 a and 5 b as well as due to theincreasing contact surface between the outwards bowing second surfacearea 3 b and the sensing surface 9 a of force detector 9, the force Fsensed by this stationary force detector 9 increases. At least a part ofthe course of force over time F(t) up to t₁ is monitored. By means ofmaximum value detection, the maximum value of force as monitored by theforce detector 9 up to t₁ is determined. Thereby and in one embodimentas shown in FIG. 3, by course (a₁) the movement characteristic of thedrives 7 a and 7 b and positioning of the stops 8 a and 8 b is selectedso that the course F(t) as monitored by force detector 9 will reach amaximum value within the time span up to t₁. Nevertheless in FIG. 3,purely quantitatively, three positive types of courses F(t) are shown as(a₁), (a₂) and (a₃). If the course substantially accords with (a₁) thereis thus determined by maximum value detection up to t₁ the valueF_(max1). This course (a₁) is encountered if the container is leaky buthas not a large leak as will be addressed later. The courses F(t)according to (a2) or (a₃) indicate that the container is either unleakyor has a small leak. If the course accords with (a2), the maximum forcevalue detected up to t₁ accords with F_(max1). If the course F(t)accords with (a₃) then the maximum force value detected up to the timet₁ is F_(max3).

Irrespective as to when the maximum force value F_(max) occurs at thetime t₁ during the time span 0 to t₁ this maximum value is detected.

If the container 1 under test has a large leak LL then the course F(t)will be as qualitatively shown by course (b) in FIG. 3. To preventfurther biasing squeeze by the biasing members 5 a and 5 b of a largeleak container 1 there is established at least one further predeterminedtime t_(LL) or time span starting with 0-time, and there is monitored atthis time t_(LL) whether the course F(t) of the container under test atleast reaches a predetermined force value F_(LL). If there is detectedthat at time t_(LL) the threshold force value F_(LL) is not reached asshown by course (b) of FIG. 3, then further biasing squeeze by themembers 5 a and 5 b is stopped before having reached their respectivestops 8 a and 8 b so as to prevent squeezing out content of thecontainer 1 through a large leak. If the container 1 under test has nota large leak LL then the maximum force value F_(max) is detected withinthe time span up to t₁ irrespective as to the time when such maximumvalue occurs and irrespective as of its absolute value. Thus differentcontainers without a large leak may lead to maximum force values F_(max)of different absolute values and such maximum values may occur duringthe time span up to t₁ at different times.

With respect to determining or detecting the maximum force value F_(max)from the force versus time course F(t) at a container under testdifferent possibilities are known to the skilled artisan. One straightahead possibility which also takes courses of the type according to (a₂)and (a₃) into account is to sample and memorize force values of courseF(t) and after t₁ has been reached to select the largest force valuewhich has been memorized. This is easily done by analogue to digitalconversion of the electric output signal of the force detector 9 andstoring in fact the force versus time course by digital samples. Furtherand following up this technique it is perfectly clear to the skilledartisan that only that part of the time course F(t) be memorized inwhich the maximum force value F_(max) is expected to occur. This area ofthe time course is shown in FIG. 3 purely as an example by area 11. Thisreduces the amount of memory necessary to determine the maximum forcevalue F_(max).

Considering the generated course (a) which defines for containerswithout large leak LL we have explained that irrespective of the type ofsuch course as of a₁ to a₃ the maximum force value F_(max) is determinedand is stored. After a predetermined time span t₂−t₁ a further forcemeasurement is made at the respectively prevailing course and thedifference of this force as measured at t₂, F(t₂) to the respectivemaximum value F_(max) is evaluated as a small leak indicative signal.Thus for the course (a₁) the difference signal ΔF is generated asindicated in FIG. 3 whereas for the courses (a₂) and (a₃) suchdifference would be zero or negative.

In FIG. 4 the inventive apparatus in its principal form which performsthe procedure as explained with the help of FIG. 3 is schematicallyshown. Thereby the same reference numbers are used as in the previousfigures with respect to features already described. The container 1 tobe tested is positioned between the biasing members 5 a and 5 b whichare driven by drives 7 a and 7 b. The stops 8 a, 8 b which have beenexplained in context with FIG. 1 are not shown in this figure. A timingunit 17 initiates the biasing movement B of the biasing members 5 a and5 b and thereby establishes with an eye on FIG. 3 for the zero time 0.The force depending electrical output signal S(F) of force detector 9 isfed at predetermined time t_(LL), controlled by the timing unit 17 asschematically shown and by switching unit SW1 to a comparator unit 21.Thus at time moment t_(LL) the output signal S(F) is compared with alarge leak indicative threshold value S_(o)(F_(LL)) as generated by unit23. Whenever at the moment t_(LL) S_(o)(F_(LL)) is not reached by theforce signal S(F), switching unit SW₂, the input thereof beingoperationally connected to S(F) is opened disabling, via a control unit25 further biasing of container 1 by the biasing members 5 a and 5 b. Ifthe threshold value S_(o)(F_(LL)) is at least reached by S(F) at themoment t_(LL), then signal S(F) is led via SW₂ to a storing unit 26which is enabled during the time span M up to the moment t₁ of FIG. 3 soas to store the values of the electric signal S(F) representing therelevant part of the characteristics F(t) as monitored by detector 9.The stored content of the storing unit 26 representing a part of thecourse F(t) up to t₁, is fed to a maximum detection and storing unit 27wherein the signal S(F_(max)) is detected and stored which signaldefines for the maximum force F_(max) which has been detected by forcedetector 9 up to the moment t₁. At the moment t₂ again controlled bytiming unit 17 the maximum value S(F_(max)) as well as the output signalprevailing at this moment t₂ at the force detector 9, S(F₂), are fed torespective inputs of a comparator unit 28 which generates at its outputan output signal OUT (Δf). The output OUT (Δf) of comparator unit 28 isindicative of leak small-behaviour of the container 1 under test.

In spite of the fact that the testing method and thus theunleaky-container manufacturing method according to the presentinvention allows detection of leaks at any part of container's wall 3 itis especially suited for detecting leaks at the most critical parts ofcontainers of the type as has been described in context with FIG. 1namely with a sealing foil-like member which is e.g. welded to theborder of the opening 4 of a bottle-like member. Such most criticalparts are the addressed welding 5 and the sealing foil-like member perse. To avoid that pressing the sealing foil-like member which forms thesecond flexible area 3 b as of FIG. 1 to the sensing surface 9 a offorce detector 9 a leak which is possibly present in the contactingsurface of the sealing foil-like member is clogged in one embodiment thesensing surface 9 a is, as schematically shown in FIG. 2 provided with asurface structure 19 which may be realized by roughening this surface toa predetermined amount. It is perfectly clear that also the contactsurfaces of the biasing members 5 a and 5 b as well as the surfacewhereupon container 1 resides may be structured to avoid also thereclogging of possibly present leaks.

Instead of evaluating directly the output signal OUT (ΔF) of comparatorunit 28 it is possible to control biasing by means of the biasingmembers 5 a and 5 b as a function of this output signal thereby removingthe stops 8 a and 8 b as of FIG. 1. Thereby a negative feedback controlloop is installed (not shown) wherein the comparator unit 28 compares arated value according to the detected and stored maximum force signalS(F_(max)) from unit 27 with an instantaneously prevailing signal S(F)and applying as an adjusting unit in the negative feedback control loopthe drives 7 a and 7 b operating the biasing members 5 a and 5 b so asto minimize the output signal OUT (ΔF) of comparator unit 28. Therebythe control signal applied to the drives 7 a and 7 b is exploited as aleak indicative signal.

In FIG. 5 one realization form of comparator unit 28 is schematicallyshown. As was addressed above memorizing the relevant part of the forceversus time representing signal S(F) as in unit 26 and determiningtherefrom the maximum value S(F_(max)) is in one embodiment performeddigitally. To do so according to FIG. 4 there is installed upstream unit26 an analogue to digital conversion unit as shown in dash lines.According to FIG. 5 the detected digital signal S(F_(max))_(#) is fed toone input of a difference forming unit 123 _(#). As schematically shownin FIG. 5 e.g. at the time t₁ or later the same stored digital signalS(F_(max))_(#) is fed also to the second input of difference formingunit 123 _(#). Thus at this moment the output of the difference formingunit 123 _(#) should be zero. If this output signal deviates from zeroit is considered as an offset signal and is stored in a storing unit 127_(#) and applied for compensation purposes to the difference formingunit 123 _(#) e.g. and as shown in FIG. 5 via an adding unit 128 _(#)upstream one of the inputs of difference forming unit 123 _(#).

At moment t₂ according to FIG. 3 the digital signal S(F₂)_(#) (seeanalogue/digital conversion upstream SW₃ in FIG. 4) is added asschematically shown in FIG. 5 by adding unit 129 _(#) to the stillprevailing signal S(F_(max))_(#). Thereby the dynamic range ofdifference forming unit 123 _(#) is fully exploited. The same principlemay also be realized in analogue signal processing technique.

In FIG. 6 there is qualitatively shown the force dependent signal S(F)at the output of force detector 9 measured at containers 1 of the sametype with the same measuring equipment which containers 1 have beenproven as unleaky. This may have been done by long term experimentsand/or with leak detecting systems which are standard and of highaccuracy but slow and/or expensive.

At t_(LL) according to FIG. 3 the force values measured at these unleakycontainers 1 are slightly different and define a statistic distribution.There results an average value (RFLL)_(m). The threshold valueS_(o)(F_(LL)) of FIG. 4 is found by substracting from the value(RFLL)_(m) an offset value ΔRFLL the magnitude of which being selectedaccording to the allowed probability that a container which has in factno large leak is treated as a container having such large leak. Thus thethreshold value S_(o)(F_(LL)) of FIG. 4 is established in one embodimentand with an eye on FIG. 6 by the value (RFLL)_(m)−ΔRFLL.

During ongoing operation on series of equal containers 1 temperature andgeometry of such containers 1 may vary latter due to manufacturingtolerance. Thereby the value (RFLL)m may slowly change. Every timeduring multiple successive testing, at the respective times t_(LL) up towhich the respective container has been identified as not heavily leaky,the actual output signal of the force detector 9 is entered into anaveraging unit 130 as shown in FIG. 7. Therein the last m values of theforce indicative signal 5(F) at t_(LL) of not heavily leaky containersare averaged. The average result signal S(F) accords with the timevarying value (RFLL) of FIG. 6. From the output average result S(F) theoffset ΔRFLL is subtracted and the result of this operation is adynamically varying reference value applied as S_(o)(F_(LL)) to unit 21according to FIG. 4. This dynamically varying reference valueS_(o)(F_(LL)) of FIG. 4 is shown in FIG. 8 qualitatively starting froman initial setting as e.g. found, as was addressed, with the help ofmeasurements at test containers 1 without large leak.

Once the container 1 under test has been found not having a large leakLL as was explained with a help of FIG. 4 there is generated at theoutput of comparator unit 28 an output signal OUT(ΔF) which isindicative for the presence of a small leak. According to FIG. 9 theoutput signal OUT(ΔF) is further evaluated by being fed to a comparatorunit 125 which is enabled at or after the time t₂. By means of areference value source 130 a reference value ΔSLREF is fed to comparatorunit 125. As will be explained later the value of ΔSLREF maycontrollably be varied in time and/or a reference value φ_(R) whichΔSLREF is referred to, may also controllably be varied in time. If thesignal OUT(ΔF) at time t₂ is larger than the reference value ΔSLREF thena signal SL is generated at unit 125 indicating the presence of a smallleak SL in the container 1 under test. If the signal OUT(ΔF) does notreach ΔSLREF then the container is considered unleaky as neither a largeleak LL nor a small leak SL has been detected, e.g. is considered as anon-leaking container, e.g. is considered as a non-leaking container.

Turning back to FIG. 8 it may be seen that the average signal S(F)(t_(LL)) is also the basis for referring ΔSLREF of FIG. 9 to. Thus inone embodiment and as shown in FIG. 9 the reference value ΔSLREF is notreferred to a static value but is referred to S(F) (t_(LL)), asgenerated at the output of averaging unit 130 of FIG.7.

In a further embodiment with features which may be realized separatelyor additionally to realizing a dynamic S_(o)(F_(LL)) and/or a dynamicbase value S(F) (t_(LL)) for ΔSLREF. Thereby and according to FIG. 10,the actual force difference signal OUT(ΔF) is led to an averaging unit135 whenever the output signal SL of comparator unit 125 of FIG. 9indicates that the container under test is unleaky. The output signal ofunit 135 which accords to an average force difference signal ΔF,averaged over the last m test cycles is offset by an amount ΔΔF theresult thereof being used as a time varying ΔSLREF signal applied atunit 125.

Looking back on FIG. 8 whereat a constant ΔSLREF signal was applied thetechnique of averaging ΔF results as schematically shown with a course(ΔSLREF)_(t) in a dynamically varying value ΔSLREF, varying according tovariations of disturbing parameters influencing such force difference.It is clear that provision of a dynamically varying (ΔSLREF)_(t) signalaccording to that representation in FIG. 8 could be realized withoutproviding a dynamically varying base value S(F) (t_(LL)) in referring(ΔSLREF)_(t) to a stable constant value φ_(R) as shown in FIG. 9 in dashrepresentations instead of referring to a dynamically varying S(F)(t_(LL)) value.

According to FIG. 11 a multitude of testing stations 140 are moved witha conveyor arrangement 142 for containers 1 to be tested. During theconveying course of the containers 1 they are brought into the testingstations 140 which keep moving with the conveyor arrangement 142. Eachtesting unit 140 is construed as has been explained. In the simplifiedrepresentation of FIG. 11 the respective squeezing biasing members 5 aand 5 b at each testing station are shown as well as the force detectors9. Without interruption of conveying and the containers 1 becomebiasingly squeezed by the biasing members 5 a and 5 b and the resultingforce on the respective force detector 9 is evaluated. If a container isdetected to be leaky it is separated from the unleaky containers asschematically shown by selecting switch 144 resulting in a train ofcontainers 1 _(UL) which are unleaky. Thus the result of containertesting is the manufacture of unleaky containers 1 ^(UL).

As force detector 9 different known detectors as e.g. Piezzo detectorsmay be used. In a today realized embodiment the force detector 9includes a resistance strain gauge sensor as e.g. of the type Z6 asmanufactured by Hottinger Baldwin Messtechnik GmbH, Germany. With thetoday's realized embodiment for inline testing a stream of plasticmaterial bottles sealingly closed with a foil-like member as wasdescribed an output rate is reached which is far above 600 bottles perminute. The extreme high output rate is primarily based on the veryquick testing method in which squeezingly biasing of the containers 1 isestablished by a quick movement of the biasing members 5 a and 5 b up totheir respective stops 8 a and 8 b as of FIG. 1. Because according toFIG. 11 more than one container, during their conveyance, are tested inparallelism the addressed high testing rate is even increased.

1. A method for manufacturing unleaky closed containers having a wallwith a flexible wall area and an opening in said wall, said openingbeing closed by a foil-like member bonded to a rim of said opening,wherein said foil-like member bows out by a bow-out distance when theinterior of said container is pressurized, comprising providing a closedcontainer; depositing said closed container opposite said foil-likemember on a support; providing a stationary force detector adjacent tosaid foil-like member at a distance which is smaller than said bow-outdistance or in contact with said foil-like member, said force detectorgenerating an output signal; biasing said flexible wall area toward theinterior of said container, thereby pressurizing said container;sampling a first force indicative signal dependent from said outputsignal at a first point in time; sampling a second force indicativesignal dependent from said output signal at a second, subsequent pointin time; generating a difference signal from said first and second forceindicative signals as a leak indicative signal and considering saidcontainer as unleaky, if said difference signal fulfils a testcriterion.
 2. The method of claim 1, further comprising performing saidbiasing by moving a biasing member toward and onto said flexible wallarea.
 3. The method of claim 1, further comprising performing saidbiasing by moving a biasing member up to a predetermined position withrespect to said container.
 4. The method of claim 1, further comprisingmonitoring a force indicative signal dependent on said output signal ata third point in time previous to said first point in time and comparingsaid force indicative signal prevailing at said third point in time witha threshold value and establishing a container as having a large leak ifsaid force indicative signal monitored does not at least reach saidthreshold value.
 5. The method of claim 1, further comprising providinga multitude of said containers moving on a conveyor.
 6. The method ofclaim 1, further comprising comparing a force indicative signaldependent from said output signal prevailing at a third point in timeprevious to said first point in time with a threshold value andestablishing a container as having no large leak if said force monitoredat said third point in time does at least reach said threshold value,averaging said force indicative signal prevailing at said third point intime if said threshold value is at least reached with such forceindicative signals monitored at tested containers considered as havingno large leak previously and providing said threshold value independency of a result of said averaging.
 7. The method of claim 1,further comprising providing at least one force indicative thresholdvalue and comparing a force indicative signal dependent on said outputsignal with said force indicative threshold value, thereby updating saidforce indicative threshold value as a function of a result of saidcomparing.
 8. The method of claim 1, further comprising stopping saidbiasing at the latest at said first point in time.
 9. The method ofclaim 8, further comprising monitoring a force indicative signaldependent from said output signal during a time span before, up to andincluding said first point in time and determining a maximum of saidforce indicative signal occurring during said time span, evaluating saidmaximum as an additional leak indicative signal.
 10. The method of claim1, further comprising comparing said difference signal with a smallleakage indicative threshold value.
 11. The method of claim 10, furthercomprising the step of averaging said difference signal with suchdifference signals generated during previous testings of containershaving been considered as unleaky and controlling said small leakageindicative threshold value in dependency of the result of saidaveraging.
 12. A method for manufacturing unleaky closed containershaving a wall with a flexible wall area and an opening in said wall,closed by a foil-like member bonded to a rim of said opening comprising:providing a closed container; depositing said closed container oppositesaid foil-like member on a support; providing a stationary forcedetector; biasing said flexible area toward the interior of saidcontainer; limiting bow-out of said foil-like member by said forcedetector and establishing said container as unleaky in dependency of anoutput signal of said detector.